Planetary gear train of automatic transmission for vehicles

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle provides at least eight forward speeds and one reverse speed by combination of: first to fourth planetary gear sets respectively having first to third rotational elements, fourth to sixth rotational elements, seventh to ninth rotational elements, or tenth to twelfth rotational elements; two transfer gears and five control elements selectively connecting one shaft to another shaft selected from first to tenth shafts or to a transmission housing. In particular, the input shaft is mounted with the first, second, and third planetary gear sets at an external circumference of the input shaft; the output shaft is mounted with the fourth planetary gear set on external circumference of the output shaft, and the tenth rotational element is externally gear-meshed with one shaft selectively connected with the second rotational element, the seventh rotational element, and the ninth rotational element respectively.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2017-0077690, filed on Jun. 20, 2017, which isincorporated herein by reference in its entirety.

FIELD

The present disclosure relates to an automatic transmission for avehicle. More particularly, the present disclosure relates to aplanetary gear train of an automatic transmission of a vehicle thatimproves power delivery performance, reduces fuel consumption, andimproves installability.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Recently, increase of oil price triggered a more competition than beforeto enhance fuel efficiency of a vehicle.

In this sense, research on an automatic transmission has been performedto simultaneously provide better drivability and fuel consumption byachieving more shift stages.

In order to achieve more shift stages for an automatic transmission, thenumber of parts is typically increased. This negatively affectsinstallability, production cost, weight and/or power flow efficiency.

An automatic transmission providing eight or more shift-stages mayeasily become lengthy due to increased components, thereby deterioratinginstallability.

In this regard, disposing planetary gear sets in parallel or employingdog clutches instead of wet-type control elements is sometimesattempted. However, we have discovered that such an arrangement may notbe widely applicable, and using dog clutches may easily deteriorateshift-feeling.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the present disclosureand therefore it may contain information that does not form the priorart that is already known to a person of ordinary skill in the art.

SUMMARY

The present disclosure provides a planetary gear train of an automatictransmission for a vehicle having advantages of realizing eight forwardspeeds and one reverse speed by a combination of four planetary gearsets, two external gears and five control elements, thereby providingimprovement of power delivery performance and fuel consumption andimproving installability by reducing a length.

In addition, a wide available range of varying gear teeth of transfergears enables easily obtaining desired gear ratios for respectivevehicles, thereby improving power delivery performance and fuelconsumption.

In one form of the present disclosure, a planetary gear train mayinclude: a first planetary gear set having first, second, and thirdrotational elements; a second planetary gear set having fourth, fifth,and sixth rotational elements; a third planetary gear set havingseventh, eighth, and ninth rotational elements; a fourth planetary gearset having tenth, eleventh, and twelfth rotational elements; an inputshaft mounted with the first, second, and third planetary gear sets atan external circumference of the input shaft; and an output shaftdisposed in parallel with the input shaft and mounted with the fourthplanetary gear set on external circumference of the output shaft,wherein the first rotational element is fixedly connected with thefourth rotational element, the second rotational element is fixedlyconnected with the input shaft, the third rotational element is fixedlyconnected with the seventh rotational element, the fifth rotationalelement is fixedly connected with the eighth rotational element, thetenth rotational element is externally gear-meshed with one shaftselectively connected with the second rotational element, the seventhrotational element, and the ninth rotational element respectively, theeleventh rotational element is fixedly connected with the output shaft,the twelfth rotational element is externally gear-meshed with the fifthrotational element, and first and second transfer gear units.

The first transfer gear unit is externally gear-meshed with at least twoshafts fixedly connected with the fifth rotational element and thetwelfth rotational element, respectively, and the second transfer gearunit is externally gear-meshed with at least two shafts fixedlyconnected with the one shaft and tenth rotational element, respectively.

The fourth rotational element and the sixth rotational element may beselectively connected with a transmission housing respectively.

An exemplary gear train may further include a first clutch selectivelyconnected with the second rotational element and the one shaft; a secondclutch selectively connected with the seventh rotational element and theone shaft; a third clutch selectively connected with the ninthrotational element and the one shaft; a first brake selectivelyconnected with the fourth rotational element and the transmissionhousing; and a second brake selectively connected with the sixthrotational element and the transmission housing.

The first transfer gear unit is arranged between the fifth rotationalelement and the twelfth rotational element; and the second transfer gearunit is arranged between the one shaft and the tenth rotational element.

The first planetary gear set may include a first sun gear as the firstrotational element, a first planet carrier as the second rotationalelement, and a first ring gear as the third rotational element. Thesecond planetary gear set may include a second sun gear as the fourthrotational element, a second planet carrier as the fifth rotationalelement, and a second ring gear as the sixth rotational element. Thethird planetary gear set may include a third sun gear as the seventhrotational element, a third planet carrier as the eighth rotationalelement, and a third ring gear as the ninth rotational element. Thefourth planetary gear set may include a fourth sun gear as the tenthrotational element, a fourth planet carrier as the eleventh rotationalelement, and a fourth ring gear as the twelfth rotational element.

According to a planetary gear train according to an exemplary form ofthe present disclosure, planetary gear sets are dividedly arranged oninput and output shafts disposed in parallel, thereby reducing a totallength of the transmission and improving its installability.

According to a planetary gear train according to an exemplary form ofthe present disclosure, at least eight forward speeds and at least onereverse speed may be realized by employing two transfer gears inaddition to a combination of planetary gear sets, thereby providing awide range of varying gear teeth so as to easily achieve desired gearratio and to easily comply with desired performance for respectivevehicles.

In addition, according to a planetary gear train according to anexemplary form of the present disclosure, a gear ratio span greater than8.7 may be achieved while realizing eight forward speeds and one reversespeed, thereby increasing an engine driving efficiency.

In addition, the linearity of step ratios of shift stages is securedwhile multi-staging the shift stage with high efficiency, thereby makingit possible to improve drivability such as acceleration before and aftera gear shift, an engine speed rhythmic sense, and the like.

In addition, the material cost can be reduced by applying the fivecontrol elements, and the drag loss can be reduced and the transmissionefficiency (fuel economy) can be increased by reducing the non-operatingcontrol element at each gear range.

Further, effects that can be obtained or expected from exemplary formsof the present disclosure are directly or suggestively described in thefollowing detailed description. That is, various effects expected fromexemplary forms of the present disclosure will be described in thefollowing detailed description.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary form of the present disclosure; and

FIG. 2 is an operational chart for respective control elements atrespective shift-stages in a planetary gear train according to anexemplary form of the present disclosure.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The present disclosure will be described more fully hereinafter withreference to the accompanying drawings, in which exemplary forms of thepresent disclosure are shown. As those skilled in the art would realize,the described forms may be modified in various different ways, allwithout departing from the spirit or scope of the present disclosure.

The drawings and description are to be regarded as illustrative innature and not restrictive, and like reference numerals designate likeelements throughout the specification.

In the following description, dividing names of components into first,second and the like is to divide the names because the names of thecomponents are the same as each other and an order thereof is notparticularly limited.

Here, the term “fixedly connected” or the like means at least twomembers are connected to each other to always rotate together.Therefore, it is to be understood by a person of an ordinary skill inthe art that the term “fixedly connected” or the like differs from theterm “operably connected” or the like.

FIG. 1 is a schematic diagram of a planetary gear train in one exemplaryform of the present disclosure.

Referring to FIG. 1, a planetary gear train includes: an input shaft IS;an output shaft OS; first, second, third, and fourth planetary gear setsPG1, PG2, PG3, and PG4; two transfer gears TF1 and TF2; and controlelements including three clutches C1, C2, and C3 and two brakes B1 andB2.

The input shaft IS is an input member and the torque from a crankshaftof an engine is input into the input shaft IS, after beingtorque-converted through a torque converter.

The output shaft OS is an output member, and arranged in parallel withthe input shaft IS so as to output a shifted driving torque to a driveshaft through a differential apparatus.

The first, second, and third planetary gear sets PG1, PG2, and PG3 arearranged at external circumference of the input shaft IS and form a mainshifting portion, and are arranged in the order of third, first, andsecond planetary gear sets PG3, PG1, and PG2 from an engine side.

The fourth planetary gear set PG4 is arranged at external circumferenceof the output shaft OS disposed in parallel with the input shaft IS andforms an auxiliary shifting portion.

The first planetary gear set PG1 is a single pinion planetary gear set,and includes a first sun gear S1, a first planet carrier PC1 thatsupports first pinion P1 externally engaged with the first sun gear S1,and a first ring gear R1 that is internally engaged with the firstpinion P1. The first sun gear S1 acts as a first rotational element N1,the first planet carrier PC1 acts as a second rotational element N2, andthe first ring gear R1 acts as a third rotational element N3.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes a second sun gear S2, a second planet carrier PC2 thatsupports second pinion P2 externally engaged with the second sun gearS2, and a second ring gear R2 that is internally engaged with the secondpinion P2. The second sun gear S2 acts as a fourth rotational elementN4, the second planet carrier PC2 acts as a fifth rotational element N5,and the second ring gear R2 acts as a sixth rotational element N6.

The third planetary gear set PG3 is a double pinion planetary gear set,and includes a third sun gear S3, a third planet carrier PC3 that isinternally gear-meshed with a third pinion P3 externally gear-meshedwith the third sun gear S3, and a third planet carrier PC3 that supportsthe third pinion P3. The third sun gear S4 acts as a seventh rotationalelement N7, the third ring gear R3 acts as an eighth rotational elementN8, and the third planet carrier PC3 acts as a ninth rotational elementN9.

The fourth planetary gear set PG4 is a single pinion planetary gear set,and includes a fourth sun gear S4, a fourth ring gear R4 that supportsfourth pinion P4 externally engaged with the fourth sun gear S4, and afourth ring gear R4 that is internally engaged with the fourth pinionP4. The fourth sun gear S4 acts as a tenth rotational element N10, thefourth ring gear R4 acts as an eleventh rotational element N11, and thefourth planet carrier PC4 acts as a twelfth rotational element N12.

In the arrangement of the first, second, and third planetary gear setsPG1, PG2, and PG3, the first rotational element N1 is fixedly connectedwith the fourth rotational element N4, the third rotational element N3is fixedly connected with the seventh rotational element N7, and thefifth rotational element N5 is fixedly connected with the eighthrotational element N8, by a corresponding shaft selected from sevenshaft TM1 to TM7.

Three shafts TM8 to TM10 are connected to the fourth planetary gear setPG4.

The ten shafts TM1 to TM10 are hereinafter described in detail.

The first shaft TM1 is fixedly connected with the first rotationalelement N1 (first sun gear S1) and the fourth rotational element N4(second sun gear S2), and is selectively connected with a transmissionhousing H thereby selectively acting as a fixed element.

The second shaft TM2 is fixedly connected with the second rotationalelement N2 (first planet carrier PC1), and is fixedly connected with theinput shaft IS thereby always acting as an input element.

The third shaft TM3 is fixedly connected with the third rotationalelement N3 (first ring gear R1) and the seventh rotational element N7(third sun gear S3).

The fourth TM4 is fixedly connected with the fifth rotational element N5(second planet carrier PC2) and the eighth rotational element N8 (thirdring gear R3).

The fifth shaft TM5 is fixedly connected with the sixth rotationalelement N6 (second ring gear R2), and is selectively connected with thetransmission housing H thereby selectively acting as a fixed element.

The sixth shaft TM6 is fixedly connected with the ninth rotationalelement N9 (third planet carrier PC3).

The seventh shaft TM7 consists of a single rotating member, and isselectively connected with the second, third, and sixth shaft TM2, TM3and TM6 respectively.

The eighth shaft TM8 is fixedly connected with the tenth rotationalelement N10 (fourth sun gear S4), and externally gear-meshed with theseventh shaft TM7.

The ninth shaft TM9 is fixedly connected with the eleventh rotationalelement N11 (fourth planet carrier PC4), and is fixedly connected withthe output shaft OS thereby always acting as an output element.

The tenth shaft TM10 is fixedly connected with the twelfth rotationalelement N12 (fourth ring gear R4), externally gear-meshed with thefourth shaft TM4.

The two transfer gears TF1 and TF2 delivers a shifted torque of the mainshifting portion having the first, second, and third planetary gear setsPG1, PG2, and PG3 to the auxiliary shifting portion having the fourthplanetary gear set PG4, in a reverse rotation.

The first transfer gear TF1 includes a first transfer drive gear TF1 afixedly connected with the fourth shaft TM4 and a first transfer drivengear TF1 b connected with the tenth shaft TM10, and externallygear-meshes the fourth shaft TM4 and the tenth shaft TM10.

The second transfer gear TF2 includes a second transfer drive gear TF2 afixedly connected with the seventh shaft TM7 and a second transferdriven gear TF2 b fixedly connected with the eighth shaft TM8, andexternally gear-meshes the seventh shaft TM7 and the eighth shaft TM8.

Respective shafts connected by the first and second transfer gears TF1and TF2 rotate in opposite directions, and the gear ratios of the firstand second transfer gears TF1 and TF2 may be preset in consideration ofdesired speed ratio of the transmission.

Here, each of the ten shafts TM1 to TM10 may be a rotational member thatfixedly connects the input and output shafts and rotational elements ofthe planetary gear sets PG1, PG2, PG3, and PG4, or may be a rotatedmembers that selectively connect the rotational elements to thetransmission housing H, or may be a fixed members that fix therotational elements to the transmission housing H.

In addition, in the above description, the term “fixedly connected” orsimilar term means that a plurality of rotational elements including theinput and output shafts, which are connected through the correspondingshafts, and the corresponding shaft are connected so as to always rotatetogether such that a plurality of fixedly connected rotational elementsand corresponding shafts may rotate in the same rotation directionand/or same number of revolutions.

In addition, in the above description, the term “selectively connected”or similar terms means that a plurality of shafts, including an inputand output shafts, are connected to each other through a couplingelement, or the corresponding shaft is fixedly connected to thetransmission housing via a coupling element. Once connected, theconnected shafts and/or rotation elements may rotate in the samerotation direction and/or same number of revolutions.

For example, when the coupling element operates to selectively connecttwo shafts to each other, the two shafts may rotate in the same rotationdirection with the same number of revolutions (i.e., same rotationspeed). Conversely, when the coupling element is released, theconnection of the two shafts is released.

Further, when the coupling element operates to selectively connect thecorresponding shaft and the transmission housing, the correspondingshaft is fixedly connected to the transmission housing. Conversely, whenthe coupling element is released, the corresponding shaft is in arotatable state.

The control elements include three clutches C1, C2, and C3 and twobrakes B1 and B2, and are arranged as follows.

The first clutch C1 is arranged between the second shaft TM2 and theseventh shaft TM7, such that the second shaft TM2 and the seventh shaftTM7 may selectively become integral.

The second clutch C2 is arranged between the third shaft TM3 and theseventh shaft TM7, such that the third shaft TM3 and the seventh shaftTM7 may selectively become integral.

The third clutch C3 is arranged between the sixth shaft TM6 and theseventh TM7, such that the sixth shaft TM6 and the seventh TM7 mayselectively become integral.

The first brake B1 is arranged between the first shaft TM1 and thetransmission housing H, such that the first shaft TM1 may selectivelyact as a fixed element.

The second brake B2 is arranged between the fifth shaft TM5 and thetransmission housing H, such that the fifth shaft TM5 may selectivelyact as a fixed element.

The respective control elements of the first, second, and third clutchesC1, C2, and C3 and the first and second brakes B1 and B2 may be realizedas multi-plate hydraulic pressure friction devices that are frictionallyengaged by hydraulic pressure. Meanwhile, the control element is any ofa dog clutch, an electric clutch, and a magnetic particle clutch, thelike which may be automatically controlled, and may be adopted.

FIG. 2 is an operational chart for respective control elements atrespective shift-stages in a planetary gear train according to anexemplary form of the present disclosure.

Referring to FIG. 2, a planetary gear train realizes shifting byoperating three control elements among the three clutches C1, C2, and C3and the two brakes B1 and B2.

[The Forward First Speed]

In the forward first speed shift-stage D1, the first clutch C1 and thefirst and second brakes B1 and B2 are simultaneously operated.

As a result, the second shaft TM2 is connected with the seventh shaftTM7 by the operation of the first clutch C1. In this state, the torqueof the input shaft IS is simultaneously input to the second and seventhshafts TM2 and TM7, and the first and fifth shafts TM1 and TM5 act asfixed elements by the operation of the first and second brakes B1 andB2, thereby realizing the forward first speed by cooperative operationof respective shafts and outputting a shifted torque to the output shaftOS fixedly connected with the ninth shaft TM9.

[The Forward Second Speed]

In the forward second speed shift-stage D2, the second clutch C2 and thefirst and second brakes B1 and B2 are simultaneously operated.

As a result, the third shaft TM3 is connected with the seventh shaft TM7by the operation of the second clutch C2. In this state, the torque ofthe input shaft IS is input to the second shaft TM2, and the first andfifth shafts TM1 and TM5 act as fixed elements by the operation of thefirst and second brakes B1 and B2, thereby realizing the forward secondspeed by cooperative operation of respective shafts and outputting ashifted torque to the output shaft OS fixedly connected with the ninthshaft TM9.

[The Forward Third Speed]

In the forward third speed shift-stage D3, the first and second clutchesC1 and C2 and the second brake B2 are simultaneously operated.

As a result, the second shaft TM2 is connected with the seventh shaftTM7 by the operation of the first clutch C1, and the third shaft TM3 isconnected with the seventh shaft TM7 by the operation of the secondclutch C2. In this state, the torque of the input shaft IS issimultaneously input to the second and seventh shafts TM2 and TM7, andthe fifth shaft TM5 acts as a fixed element by the operation of thesecond brake B2, thereby realizing the forward third speed bycooperative operation of respective shafts and outputting a shiftedtorque to the output shaft OS fixedly connected with the ninth shaftTM9.

[The Forward Fourth Speed]

In the forward fourth speed shift-stage D4, the second and third firstclutch C2 and C3 and the second brake B2 are simultaneously operated.

As a result, the third shaft TM3 is connected with the seventh shaft TM7by the operation of the second clutch C2, and the sixth shaft TM6 isconnected with the seventh shaft TM7 by the operation of the thirdclutch C3. In this state, the torque of the input shaft IS is input tothe second shaft TM2, and the fifth shaft TM5 acts as a fixed element bythe operation of the second brake B2, thereby realizing the forwardfourth speed by cooperative operation of respective shafts andoutputting a shifted torque to the output shaft OS fixedly connectedwith the ninth shaft TM9.

[The Forward Fifth Speed]

In the forward fifth speed shift-stage D5, the first and third clutchesC1 and C3 and the second brake B2 are simultaneously operated.

As a result, the second shaft TM2 is connected with the seventh shaftTM7 by the operation of the first clutch C1, and the sixth shaft TM6 isconnected with the seventh shaft TM7 by the operation of the thirdclutch C3. In this state, the torque of the input shaft IS issimultaneously input to the second and seventh shafts TM2 and TM7, andthe fifth shaft TM5 acts as a fixed element by the operation of thesecond brake B2, thereby realizing the forward fifth speed bycooperative operation of respective shafts and outputting a shiftedtorque to the output shaft OS fixedly connected with the ninth shaftTM9.

[The Forward Sixth Speed]

In the forward sixth speed shift-stage D6, the first, second, and thirdclutches C1, C2, and C3 are simultaneously operated.

As a result, the second shaft TM2 is connected with the seventh shaftTM7 by the operation of the first clutch C1, the third shaft TM3 isconnected with the seventh shaft TM7 by the operation of the secondclutch C2, and the sixth shaft TM6 is connected with the seventh shaftTM7 by the operation of the third clutch C3. In this state, the torqueof the input shaft IS is simultaneously input to the second and seventhshafts TM2 and TM7, thereby realizing the forward sixth speed thatoutputs a torque as inputted, by cooperative operation of respectiveshafts and outputting a shifted torque to the output shaft OS fixedlyconnected with the ninth shaft TM9.

[The Forward Seventh Speed]

In the forward seventh speed shift-stage D7, the first and thirdclutches C1 and C3 and the first brake B1 are simultaneously operated.

As a result, the second shaft TM2 is connected with the seventh shaftTM7 by the operation of the first clutch C1, and the sixth shaft TM6 isconnected with the seventh shaft TM7 by the operation of the thirdclutch C3. In this state, the torque of the input shaft IS issimultaneously input to the second and seventh shafts TM2 and TM7, andthe first shaft TM1 acts as a fixed element by the operation of thefirst brake B1, thereby realizing the forward seventh speed bycooperative operation of respective shafts and outputting a shiftedtorque to the output shaft OS fixedly connected with the ninth shaftTM9.

[The Forward Eighth Speed]

In the forward eighth speed shift-stage D8, the second and thirdclutches C2 and C3 and the first brake B1 are simultaneously operated.

As a result, the third shaft TM3 is connected with the seventh shaft TM7by the operation of the second clutch C2, and the sixth shaft TM6 isconnected with the seventh shaft TM7 by the operation of the thirdclutch C3. In this state, the torque of the input shaft IS is input tothe second shaft TM2, and the first shaft TM1 acts as a fixed element bythe operation of the first brake B1, thereby realizing the forwardeighth speed by cooperative operation of respective shafts andoutputting a shifted torque to the output shaft OS fixedly connectedwith the ninth shaft TM9.

[The Reverse Speed]

In the reverse speed, the third clutch C3 and first and second brakes B1and B2 are simultaneously operated.

As a result, the sixth shaft TM6 is connected with the seventh shaft TM7by the operation of the third clutch C3. In this state, the torque ofthe input shaft IS is input to the second shaft TM2, and the first andfifth shafts TM1 and TM5 act as a fixed element by the operation of thefirst and second brakes B1 and B2, thereby realizing the reverse speedby cooperative operation of respective shafts and outputting a shiftedtorque to the output shaft OS fixedly connected with the ninth shaftTM9.

As described above, according to a planetary gear train according to anexemplary form of the present disclosure, eight forward speeds and onereverse speed may be realized by a combination of four planetary gearsets PG1, PG2, PG3, and PG4, two transfer gears TF1 and TF2, and fivecontrol elements, thereby providing improvement of power deliveryperformance and fuel consumption and improving installability byshortening the length of an automatic transmission.

In addition, according to a planetary gear train according to anexemplary form of the present disclosure, two transfer gears of externalgears arranged on the output shaft OS are employed in addition to threeplanetary gear sets, and thus, gear teeth may be widely varied so as toeasily achieve desired gear ratio and to easily comply with desiredperformance for respective vehicles.

In addition, according to a planetary gear train according to anexemplary form of the present disclosure, a gear ratio span greater than8.7 may be achieved while realizing eight forward speeds and one reversespeed, thereby increasing an engine driving efficiency.

In addition, the linearity of step ratios of shift stages is securedwhile multi-staging the shift stage with high efficiency, thereby makingit possible to improve drivability such as acceleration before and aftera gear shift, an engine speed rhythmic sense, and the like.

In addition, the material cost can be reduced by applying the fivecontrol elements, and the drag loss can be reduced and the transmissionefficiency (fuel economy) can be increased by reducing the non-operatingcontrol element at each gear range.

While this present disclosure has been described in connection with whatis presently considered to be practical exemplary forms, it is to beunderstood that the present disclosure is not limited to the disclosedforms, but, on the contrary, is intended to cover various modificationsand equivalent arrangements included within the spirit and scope of thepresent disclosure.

DESCRIPTION OF SYMBOLS

-   PG1,PG2,PG3,PG4 . . . first, second, third, and fourth planetary    gear sets-   S1,S2,S3,S4 . . . first, second, third, and fourth sun gears-   PC1,PC2,PC3,PC4 . . . first, second, third, and fourth planet    carriers-   R1,R2,R3,R4 . . . first, second, third, and fourth ring gears-   IS . . . input shaft-   OS . . . output shaft-   B1,B2 . . . first, and second third brakes-   C1,C2,C3 . . . first, second, and third clutches-   TF1,TF2 . . . first and second transfer gears-   TM1,TM2,TM3,TM4,TM5,TM6,TM7,TM8,TM9,TM10 . . . first, second, third,    fourth, fifth, sixth, seventh, eighth, ninth, and tenth shaft

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle, the planetary gear train comprising: a firstplanetary gear set having first, second, and third rotational elements;a second planetary gear set having fourth, fifth, and sixth rotationalelements; a third planetary gear set having seventh, eighth, and ninthrotational elements; a fourth planetary gear set having tenth, eleventh,and twelfth rotational elements; an input shaft mounted with the first,second, and third planetary gear sets at an external circumference ofthe input shaft; and an output shaft disposed in parallel with the inputshaft and mounted with the fourth planetary gear set on externalcircumference of the output shaft, wherein the first rotational elementis fixedly connected with the fourth rotational element, the secondrotational element is fixedly connected with the input shaft, the thirdrotational element is fixedly connected with the seventh rotationalelement, the fifth rotational element is fixedly connected with theeighth rotational element, the tenth rotational element is externallygear-meshed with one shaft selectively connected with the secondrotational element, the seventh rotational element, and the ninthrotational element respectively, the eleventh rotational element isfixedly connected with the output shaft, the twelfth rotational elementis externally gear-meshed with the fifth rotational element, and firstand second transfer gear units, wherein the first transfer gear unit isexternally gear-meshed with at least two shafts respectively and fixedlyconnected with the fifth rotational element and the twelfth rotationalelement, and the second transfer gear unit is externally gear-meshedwith at least two shafts respectively and externally gear-meshed withthe one shaft and tenth rotational element.
 2. The planetary gear trainof claim 1, wherein the fourth rotational element and the sixthrotational element are selectively connected with a transmission housingrespectively.
 3. The planetary gear train of claim 2, furthercomprising: a first clutch selectively connected with the secondrotational element and the one shaft; a second clutch selectivelyconnected with the seventh rotational element and the one shaft; a thirdclutch selectively connected with the ninth rotational element and theone shaft; a first brake selectively connected with the fourthrotational element and the transmission housing; and a second brakeselectively connected with the sixth rotational element and thetransmission housing.
 4. The planetary gear train of claim 1, whereinthe first transfer gear unit is arranged between the fifth rotationalelement and the twelfth rotational element; and the second transfer gearunit is arranged between the one shaft and the tenth rotational element.5. The planetary gear train of claim 1, wherein the first planetary gearset comprises a first sun gear as the first rotational element, a firstplanet carrier as the second rotational element, and a first ring gearas the third rotational element; the second planetary gear set comprisesa second sun gear as the fourth rotational element, a second planetcarrier as the fifth rotational element, and a second ring gear as thesixth rotational element; the third planetary gear set comprises a thirdsun gear as the seventh rotational element, a third planet carrier asthe eighth rotational element, and a third ring gear as the ninthrotational element; and the fourth planetary gear set comprises a fourthsun gear as the tenth rotational element, a fourth planet carrier as theeleventh rotational element, and a fourth ring gear as the twelfthrotational element.
 6. A planetary gear train of an automatictransmission for a vehicle, the planetary gear train comprising: a firstplanetary gear set having first, second, and third rotational elements;a second planetary gear set having fourth, fifth, and sixth rotationalelements; a third planetary gear set having seventh, eighth, and ninthrotational elements; a fourth planetary gear set having tenth, eleventh,and twelfth rotational elements; an input shaft mounted with the firstand second planetary gear sets at an external circumference of the inputshaft; and an output shaft disposed in parallel with the input shaft andmounted with the third, fourth planetary gear set on externalcircumference of the output shaft; a first shaft fixedly connected withthe first rotational element and the fourth rotational element; a secondshaft fixedly connected with the second rotational element and the inputshaft; a third shaft fixedly connected with the third rotational elementand the seventh rotational element; a fourth shaft fixedly connectedwith the fifth rotational element and the eighth rotational element; afifth shaft fixedly connected with the sixth rotational element; a sixthshaft fixedly connected with the ninth rotational element; a seventhshaft selectively connected with the second, third, and sixth shaftsrespectively; an eighth shaft fixedly connected with the tenthrotational element, and externally gear-meshed with the seventh shaft; aninth shaft fixedly connected with the eleventh rotational element andthe output shaft; a tenth shaft fixedly connected with the twelfthrotational element, and externally gear-meshed with the fourth shaft;and first and second transfer gear units, wherein the first transfergear unit is externally gear-meshed with the fourth shaft and the tenthshaft, and the second transfer gear unit is externally gear-meshed withthe seventh shaft and the eighth shaft.
 7. The planetary gear train ofclaim 6, wherein the seventh shaft is selectively connected with thesecond, third, and sixth shafts respectively, and the first shaft andthe fifth shaft are selectively connected with a transmission housingrespectively.
 8. The planetary gear train of claim 7, furthercomprising: a first clutch arranged between the second shaft and theseventh shaft; a second clutch arranged between the third shaft and theseventh shaft; a third clutch arranged between the sixth shaft and theseventh shaft; a first brake arranged between the first shaft and thetransmission housing; and a second brake arranged between the fifthshaft and the transmission housing.
 9. The planetary gear train of claim6, wherein the first transfer gear unit is arranged between the fourthshaft and the tenth shaft; and the second transfer gear unit is arrangedbetween the seventh shaft and the eighth shaft.
 10. The planetary geartrain of claim 6, wherein the first planetary gear set comprises a firstsun gear as the first rotational element, a first planet carrier as thesecond rotational element, and a first ring gear as the third rotationalelement; the second planetary gear set comprises a second sun gear asthe fourth rotational element, a second planet carrier as the fifthrotational element, and a second ring gear as the sixth rotationalelement; the third planetary gear set comprises a third sun gear as theseventh rotational element, a third planet carrier as the eighthrotational element, and a third ring gear as the ninth rotationalelement; and the fourth planetary gear set comprises a fourth sun gearas the tenth rotational element, a fourth planet carrier as the eleventhrotational element, and a fourth ring gear as the twelfth rotationalelement.